The present invention relates generally to the high-voltage stability of computed tomography x-ray sources. More particularly, the present invention relates to the minimization of electrostatic field line bending within the triple point areas of an x-ray tube.
High-voltage stability of high power and high-voltage computed tomography (CT) x-ray sources, such as an x-ray tube, is essential to constructing, seasoning, testing, and placing of the x-ray sources in service. During manufacturing of an x-ray tube, the x-ray tube is assembled and tested. Following the manufacturing of the x-ray tube, the x-ray tube is further tested and calibrated during system assembly. Many of the test protocols and calibration procedures are more aggressive than the typical or anticipated protocols and procedures in actual endpoint customer use. A desire to withstand the rigorous protocols and procedures in addition to a desire for the quick and efficient execution thereof, results in a need for a highly robust x-ray source that satisfies rigorous high-voltage x-ray tube design requirements.
In single-ended or monopolar high-voltage x-ray tubes x-rays are generated by accelerating an electron beam across a vacuum gap between a cathode and a rotating anode. The cathode and the anode reside within a vacuum vessel, which is sometimes referred to as an insert or frame. High voltage is supplied to the cathode via a high voltage cable through a single high voltage insulator. In the case of anode-grounded x-ray tubes, the high voltage insulator can be at a negative potential with respect to the potential of a ground reference.
The high-voltage insulator isolates and separates the cathode from the walls of the insert, which are often approximately at the ground potential. In so doing, the insulator provides a vacuum seal between the cathode and the walls. The high-voltage cable penetrates the insert or vacuum vessel, via conductor pins, to provide high-voltage to the cathode. The high-voltage cable is coupled to the insert by a connector having a Faraday cage. The Faraday cage is typically in the form of a cylinder that encompasses and prevents high-voltage stress on and breakdown of the conductor pins, which provide conduction between the high-voltage cable and the cathode.
There are generally two main design features that aid in the high-voltage stability of the insert. The two main features are the design of a vacuum side and of an atmospheric-side of the high-voltage insulator. Vacuum-tight sealing techniques are used on the vacuum side of the insulator to prevent atmospheric gas leakage into the x-ray tube. The atmospheric-side includes the use of the connector having the Faraday cage. Since the connector is typically at ground potential, the Faraday cage is used to isolate and separate the conductor pins and the connector.
The insulator designs are hybrid in nature. The insulator provides high-voltage potential isolation and separation through use of air gaps and insulating material. The insulator also provides mechanical strength to maintain certain physical distances to sub-millimeter tolerances over a wide range of temperatures. The insulator provides a solid surface for the establishment of electrostatic potential, across which arcing can occur. The arc path may, for example, exist between a pair of high-voltage terminals, such as between the cathode and the insert walls.
The areas within the vacuum vessel along which the conductors and the insulator are adjacent to or are in contact with each other are referred to collectively as “triple point areas”. High electric field stress is experienced both externally from and internally to the insulator near the cathode and conductors in the triple point areas.
The high electric field stress in the triple point areas can produce punctures in the insulator and electron emission through field emission effects and other hybrid microscopic mechanisms. Once the charges from the electron emission are separated from a solid surface, such as the cathode, and reside within the vacuum or the insulator they can accelerate under the effects of the electric fields and cascade to initiate arcs. The arcs can occur along the above stated paths. The arcing can damage, breakdown, and cause cracking of the insulator. Breakdown of the insulator can eventually cause air leaks and render the x-ray tube inoperable. The arcing can also result in atmosphere side flashovers, which can cause damage to other x-ray system componentry.
Thus, there exists a need for an improved x-ray tube design that minimizes high electric field stresses experienced within the triple point areas, while maintaining and satisfying present voltage potential differences and electric field performance standards and tolerances of an x-ray tube.